1. Field of the Invention
The present invention relates to a system for and a method of interconnecting a first high-frequency transmission line such as a multilayer circuit board having a stripline or a microstrip line, for example, and a second high-frequency transmission line such as a coaxial cable or a multilayer circuit board, for example, while providing a good impedance matching therebetween.
2. Description of the Related Art
The recent technological advances in IT (Information Technology) have resulted in a massive growth of information communication networks processing electric signals in a frequency range from several hundreds MHz to GHz. For example, radio waves in the GHz frequency band are used by information and communication devices and instruments such as cellular phones and wireless LAN (Local Area Network) terminals and the ITS (Intelligent Transportation System). To meet increasing demands for more and more radio waves for many different kinds of communication devices, these devices and instruments are expanded by higher frequencies from several GHz to ten GHz and higher.
For interconnecting parts inside high-frequency devices and/or instruments and also interconnections between high-frequency devices and instruments, it is necessary to connect two different high-frequency transmission lines to each other at many locations.
High-frequency devices and instruments or high-frequency transmission lines on multilayer circuit boards are interconnected usually by high-frequency coaxial connectors for input/output of signals. However, using high-frequency coaxial connectors tends to increase the system cost.
FIG. 23 of the accompanying drawings shows in exploded perspective a connection structure according to prior art 1 for interconnecting a coaxial cable 6 and a multilayer circuit board 2 having a stripline thereon without any coaxial connector in use, as disclosed in Japanese laid-open patent publication No. 2001-102817.
In FIG. 23, the multilayer circuit board 2 has conductive layers shown shaded. In some of the accompanying drawings, all conductive layers of circuit boards are shown shaded for distinguishing themselves clearly.
FIG. 24 of the accompanying drawings shows in perspective, partly broken away, a portion of the connection structure illustrated in FIG. 23.
In FIGS. 23 and 24, a central conductor 8 of a coaxial cable 6 is inserted into a via hole 4 in a multilayer circuit board 2 having a triplate stripline disposed thereon, and electrically connected to the via hole 4 by a solder body 10. The multilayer circuit board 2 has an upper ground pattern (ground conductor) 12 electrically connected to an outer conductor 14 of the coaxial cable 6 by a solder body 16. The multilayer circuit board 2 also has a lower ground pattern 20 electrically connected to the upper ground pattern 12 by a ground via hole 22.
The term “via hole” used herein means a plated through hole with lands, interconnecting conductive layers of a multilayer circuit board.
With the connection structure shown in FIGS. 23 and 24, the central conductor 8 of the coaxial cable 6 and a signal conductor 24 which serves as a signal line of the multilayer circuit board 2 are electrically connected to each other, and the outer conductor 14 of the coaxial cable 6 and the ground patterns 12, 20 of the multilayer circuit board 2 are electrically connected to each other.
FIG. 25 of the accompanying drawings shows in exploded perspective a connection structure according to prior art 2 for interconnecting a coaxial cable 6 and a multilayer circuit board 32 having a microstrip line thereon without any coaxial connector in use.
FIG. 26 of the accompanying drawings shows in perspective, partly broken away, a portion of the connection structure illustrated in FIG. 25.
As shown in FIGS. 25 and 26, a strip end of a central conductor 8 of a coaxial cable 6 is electrically connected by a solder body 36 to the pattern of a signal conductor 34 of a multilayer circuit board 32. The multilayer circuit board 32 has a ground pattern 38 electrically connected to an outer conductor 14 of the coaxial cable 6 by a solder body 16. The multilayer circuit board 2 also has another ground pattern 40 electrically connected to the ground pattern 38 by ground via holes 42.
According to the prior art 1 and the prior art 2, no expensive coaxial connector is used to interconnect the coaxial cable 6 and the signal conductor 24 and the ground 12 of the multilayer circuit board 2 or the coaxial cable 6 and the signal conductor 34 and the ground 38 of the multilayer circuit board 32.
However, the high-frequency transmission line connecting systems according to the prior art 1 and the prior art 2 have various problems as described below.
First, the high-frequency transmission line connecting system according to the prior art 1 shown in FIGS. 23 and 24 has the following drawbacks:
Since the central conductor 8 of the coaxial cable 6 needs a certain length of its bare wire strip to be soldered, the bare wire strip holds a large stray inductance which leads to an impedance mismatch, and also causes a large reflection loss.
Because the multilayer circuit board 2 has the via hole 4 in which the central conductor 8 of the coaxial cable 6 is inserted, a large stray capacitance is also formed between the via hole 4 and the ground patterns in the vicinity of the via hole 4. The large stray capacitance also leads to an impedance mismatch and gives rise to a large reflection loss.
The central conductor 8 of the coaxial cable 6 should be bent at a right angle for insertion into the via hole 4. Therefore, it is likely for the central conductor 8 to reveal a various different bending shape. It is also difficult to keep the bare wire strip of the central conductor 8 constant in length in mounting the coaxial cable 6 on the multilayer circuit board 2. As a result, it is difficult to keep the connection between the coaxial cable 6 and the multilayer circuit board 2 in constant physical configuration and to keep constant electrical characteristics. Stated otherwise, skill and expertise are required to make the connection between the coaxial cable 6 and the multilayer circuit board 2 in constant physical configuration.
Similarly, the central conductor 8 and the via hole 4 need to be soldered with skill and expertise because the space available for soldering is not enough.
The coaxial cable 6, because of its cylindrical shape, is free to move on the flat surface of the multiplayer circuit board 2. Therefore, there is difficulty in positioning and soldering the coaxial cable 6. It is also difficult to keep the connection between the coaxial cable 6 and the multilayer circuit board 2 in a uniform interconnection quality. The fixation of the coaxial cable 6 is relatively weak in mechanical strength.
The connection structure according to the prior art 1 can be used within a frequency range of at least several GHz.
The high-frequency transmission line connecting system according to the prior art 2 has had the following disadvantages:
As there is no via hole in the multilayer circuit board 32, any stray capacitance produced by the connection structure is much smaller than with the prior art 1, allowing an impedance matching to be achieved with greater ease. However, since solder is used to connect the central conductor 8 and the signal conductor 34 to each other without any fixing mechanism, intricate workmanship is required to apply an appropriate amount of solder to an appropriate area for making reliable and economical solder joints. Stated otherwise, the efficiency with which the connection structure is assembled is low, and the time required to assemble the connection structure is long, resulting in a reduced yield.
According to the prior art 2, as with the prior art 1, inasmuch as the coaxial cable 6 is required to be positioned on and fixed to the flat surface of the multilayer circuit board 2, difficulty has been encountered in positioning and soldering the coaxial cable 6, making it difficult to maintain a uniform interconnection quality. Furthermore, because the central conductor 8 is soldered to the signal conductor 34 of the multilayer circuit board 32, the fixation of the central conductor 8 soldered to the signal conductor 34 is of relatively weak mechanical strength.